J. Phys. Chem. A 2010, 114, 10929–10935
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Photochemistry of Linear-Shaped Phenylacetylenyl- and (Phenylacetylenyl)phenylacetylenylSubstituted Aromatic Enediynes Yoko Sugiyama,† Yoshihiro Shinohara,‡ Atsuya Momotake,† Kayori Takahashi,§ Yoko Kanna,| Yoshinobu Nishimura,† and Tatsuo Arai*,† Graduate School of Pure and Applied Sciences and Research Facility Center for Science and Technology, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan, National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba City, Ibaraki 305-8565, Japan, and Faculty of Science, UniVersity of the Ryukyus, Nishihara, Okinawa 903-0213, Japan ReceiVed: June 15, 2010; ReVised Manuscript ReceiVed: September 12, 2010
The series of linear-shaped phenylacetylenyl- and (phenylacetylenyl)phenylacetylenyl-substituted aromatic enediynes 1-3 were synthesized as pure trans and cis isomers and their photochemistry explored. With expansion of the π-electron system, the absorption spectra red-shifted and the molar extinction coefficients dramatically increased up to 122000 M-1 cm-1 for trans-3. The absorption spectra of cis-2 and cis-3 consisted of two independent absorption bands. The fluorescence quantum yields of the molecules were high, even for the cis isomers (Φf ) 0.39-0.61). The fluorescence decay of each of the compounds was analyzed as a single exponential and the wavelength dependence of time constants was not observed, indicating a single emitting state in all cases. All isomers exhibited mutual cis-trans photoisomerization. The quantum yield of both trans-to-cis and cis-to-trans photoisomerization considerably decreased in 2 and 3, presumably due to an increased number of photochemical processes that yield nonreactive excited species and which result in nonradiative deactivation. Three energy minima exist in the excited triplet state, where the energy of planar conformation decreased with the extension of the phenyl acetylenyl chain, resulting in the promotion of nonradiative processes without conformational change. Introduction Enediynes have been widely investigated, especially in the area of antitumor antibiotics. The drugs esperamicin, calichemicin, and dynemicin, for example, all possess an aliphatic endiyne unit and produce a reactive 1,4-biradical aromatic intermediate upon heating.1-4 Aromatic enediynes, on the other hand, are usually thermally stable and have been known to exhibit unique photochemical properties in organic solvents. Previously we reported the photochemical reactivity of aromatic enediyne bisphenylethynylethene (BEE) (Figure 1a).5 BEE gives efficient fluorescence emission from both the cis and trans isomers and undergoes intersystem crossing in addition to cis-trans photoisomerization. Furthermore, three energy minima exist in the triplet state potential surface where the planar triplet state (3trans* and 3cis*) and the perpendicular triplet state (3p*) are equilibrated. The fluorescence spectra, Stokes shift, fluorescence lifetime, fluorescence quantum yield, and quantum yield of transto-cis photoisomerization for the “push-pull” aromatic enediynes trans- and cis-1-(4-dimethylaminophenyl)-6-(4-nitrophenyl)hex-3-ene-1,5-diynes (DANE) that contain both electronwithdrawing and electron-donating groups on the phenyl rings (Figure 1b) were found to exhibit a strong dependence on solvent polarity in the less-polar region.6 The effects of methoxy substituents on the phenyl ring of the cis isomer of enediynes were also explored; ortho-substituted compounds (Figure 1b) * To whom correspondence should be addressed. † Graduate School of Pure and Applied Sciences, University of Tsukuba. ‡ Research Facility Center for Science and Technology, University of Tsukuba. § National Institute of Advanced Industrial Science and Technology (AIST). | Faculty of Science, University of the Ryukyus.
showed a red shift of the fluorescence even though crystallographic analysis suggests that the single bond connecting the phenyl ring and carbon-carbon triple bond takes a twisted conformation.7 Furthermore, both lipophilic8 and amphiphilic9 enediyne-cored dendrimers were prepared as pure cis and trans isomers. All dendrimers were fluorescent with fluorescence quantum yields of 0.10-0.66 and underwent efficient photochemical isomerization in THF with quantum yields of 0.18-0.50. Fluorescence and triplet lifetimes were strongly affected by dendrimer generation and solvent polarity. On the basis of these results, we focused our attention on the photochemical properties of the linear-shaped aromatic enediynes 1-3, where the π-electron system of the enediyne should be expanded due to introduction of phenylacetylenyl or (phenylacetylenyl)phenylacetylenyl units in the para position (Figure 1c). The effects of extending the π-electron system on the photochemistry of aromatic enediynes have not been studied but are of interest from the viewpoint of construction of molecules with new emissive and/or reactive properties. Experimental Section Apparatus. Sample solutions were prepared in benzene (Kanto Chemical) and deoxygenated by bubbling highly purified argon (>99.999%) through a needle. Fluorescence and excitation spectra were measured with an Hitachi F-4500 fluorescence spectrophotometer using a 1 cm ×1 cm quartz cuvette. Absorption spectra were recorded with a Shimadzu UV-1600 spectrophotometer. Fluorescence decay measurements were performed using a time-correlated single-photon counting method. The apparatus was assembled based on the method previously described.10,11 Excitation at 410 nm was achieved using a diode laser (PicoQuant, LDH-P-C- 405) driven by a power control
10.1021/jp1054827 2010 American Chemical Society Published on Web 09/27/2010
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Figure 1. Chemical structures of aromatic enediynes.
unit (PicoQuant, PDL 800-B) with a repetition rate of 2.5 MHz. Temporal profiles of fluorescence decay were recorded using a microchannel plate photomultiplier (Hamamatsu, R3809U) equipped with a TCSPC computer board module (Becker and Hickl, SPC630). The full width at half-maximum (fwhm) of the instrument response function was 51 ps. Criteria for the best fit included χ2 values and Durbin-Watson parameters obtained by nonlinear regression.12 Materials. cis-1,6-Bis(4-tert-butylphenyl)hexa-3-en-1,5diyne (cis-1). A solution of cis-1,2-dichloroethylene (456 mg, 4.71 mmol) and 1-tert-butyl-4-ethynylbenzene (2.43 g, 15.4 mmol) in benzene was added to a mixture of Pd(PPh3)4 (336 mg, 29 µmol), CuI (109 mg, 0.57 mmol), and n-BuNH2 (1.15 g, 15.7 mmol) in 20 mL of deaerated benzene which was cooled at -78 °C. The mixture was allowed to warm to room temperature and was stirred for 14 h. The reaction was then quenched by addition of water and extracted with dichloromethane. The organic layer was washed with brine, dried over MgSO4, filtered, and evaporated. The residue was purified by silica gel column chromatography (eluent, hexane/chloroform (80/20)) to give cis-1 as a pale yellow solid (1.08 g, 59%). 1H NMR (270 MHz, CDCl3): 7.46 (d, J ) 8.1 Hz, 4H), 7.35 (d, J ) 8.1 Hz, 4H), 6.02 (s, 2H), 1.32 (s, 9H). 13C NMR (270 MHz, CDCl3): 31.2, 34.9, 86.8, 97.7, 119.1, 120.1, 125.3, 131.2, 151.7. Anal. Calcd for C26H28: C, 91.71; H, 8.29. Found: C, 91.57; H, 8.29. trans-1,6-Bis(4-tert-butylphenyl)hexa-3-en-1,5-diyne (trans1). A benzene solution of trans-1,2-dichloroethylene (300 mg, 3.0 mmol) and 1-tert-butyl-4-ethynylbenzene (1.2 g, 7.0 mmol) was added to a mixture of Pd(PPh3)4 (420 mg, 0.36 mmol), CuI (95 mg, 0.50 mmol), and n-BuNH2 (650 mg, 9.0 mmol) in 30 mL of deaerated benzene which was cooled at -78 °C. The mixture was allowed to warm to room temperature and then stirred for 14 h, after which time the reaction was quenched by addition of water and then extracted with dichloromethane. The
organic layer was washed with brine, dried over MgSO4, filtered, and evaporated. The residue was purified by silica gel column chromatography (eluent, hexane/chloroform (90/10)) to give trans-1 as a white powder (67 mg, 5%). 1H NMR (270 MHz, CDCl3): 7.41-7.32 (m, 8H), 6.26 (s, 2H), 1.33 (s, 18H). 13C NMR (67.5 MHz, CDCl3): 31.2, 34.9, 87.6, 94.9, 120.0, 120.4, 125.3, 131.2, 151.8. Anal. Calcd for C26H28: C, 91.71; H, 8.29. Found: C, 91.94; H, 8.35. cis-1,6-Bis(4-(2-(4-tert-butylphenyl)ethynyl)phenyl)hexa-3en-1,5-diyne (cis-2). A benzene solution of cis-1,2-dichloroethylene (97 mg, 1.0 mmol) and 1-(2-(4-tert-butylphenyl)ethynyl)4-ethynylbenzene13 (580 mg, 2.2 mmol) was added to a mixture of Pd(PPh3)4 (56 mg, 0.05 mmol), CuI (27 mg, 0.14 mmol), and n-BuNH2 (217 mg, 3.0 mmol) in 20 mL of deaerated benzene which was cooled at -78 °C. The mixture was allowed to warm to room temperature and then stirred for 14 h, after which time the reaction was quenched by addition of water and extracted with dichloromethane. The organic layer was washed with brine, dried over MgSO4, filtered, and evaporated. The residue was purified by silica gel column chromatography (eluent, hexane/chloroform (80/20)) to give cis-2 as a white powder (110 mg, 20%). 1H NMR (270 MHz CDCl3): 7.48-7.45 (m, 12H), 7.39-7.36 (m, 4H), 6.12 (s, 2H), 1.33 (s, 18H). 13C NMR (67.5 MHz, CDCl3): 31.2, 34.9, 88.4, 89.0, 91.8, 97.6, 119.5, 119.8, 122.5, 123.8, 125.3, 131.3, 131.5, 131.5, 151.7. Anal. Calcd for C42H36: C, 92.98; H, 6.78. Found: C, 93.29; H, 6.71. trans-1,6-Bis(4-(2-(4-tert-butylphenyl)ethynyl)phenyl)hexa3-en-1,5-diyne (trans-2). A solution of cis-2 (50.0 mg, 0.09 mmol) in 20 mL of chloroform was exposed to a hand-held UV lamp (365 nm) for 12 h. After evaporation, the residue was purified by GPC on a TSKgel G1000H column (TOHSO) by elution with hexane to give trans-2 as a white powder (9.4 mg, 19%). 1H NMR (270 MHz,CDCl3): 7.49 (d, J ) 8.1 Hz, 4H), 7.47 (d, J ) 8.1 Hz, 4H), 7.43 (d, J ) 8.1 Hz, 4H), 7.38 (d, J
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Figure 2. Absorption spectra of the (a) trans and (b) cis isomers of BEE (thin line), 1 (open circles), 2 (closed circles), and 3 (solid line) in benzene under argon. Fluorescence spectra of the (c) trans and (d) cis isomers of BEE (thin line), 1 (open circles), 2 (closed circles), and 3 (solid line) in benzene under argon.
) 8.1 Hz, 4H), 6.30 (s, 2H), 1.33 (s, 18H). MALDI-TOFMS (m/z) [M]+ calcd for C42H36, 540.28; found, 540.01. cis-1,6-Bis(4-(2-(4-(2-(3,5-di-tert-butylphenyl)ethynyl)phenyl)ethynyl)phenyl)hexa-3-en-1,5-diyne (cis-3). A benzene solution of cis-1,2-dichloroethylene (20 mg, 0.2 mmol) and 1,3-di-tert-butyl-5-(2-(4-(2-(4-ethynylphenyl)ethynyl)phenyl)ethynyl)benzene13 (203 mg, 0.49 mmol) was added to a mixture of Pd(PPh3)2Cl2 (23 mg, 0.03 mmol), CuI (9.1 mg, 0.04 mmol), and n-BuNH2 (50 mg, 0.68 mmol) in 10 mL of deaerated benzene which was cooled at -78 °C. The mixture was allowed to warm to room temperature and stirred for 5 h. After evaporation, the residue was purified by silica gel column chromatography (eluent, hexane/chloroform (85/15)) to give cis-3 as a white powder (28 mg, 16%). 1H NMR (400 MHz,CDCl3): 7.54-7.46 (m, 16H), 7.41-7.40 (m, 2H), 7.38-7.37 (m, 4H), 6.12 (s, 2H), 1.33 (s, 36H). 13C NMR (67.5 MHz, CDCl3): 150.8, 131.5, 131.5, 131.5, 131.4, 125.8, 123.6, 122.9, 122.9, 122.4, 121.8, 119.6, 97.5, 92.6, 91.4, 90.7, 89.2, 87.8, 34.9, 31.4. MALDI-TOFMS (m/z) [M + H]+ calcd for C66H60, 853.47; found, 853.47. trans-1,6-Bis(4-(2-(4-(2-(3,5-di-tert-butylphenyl)ethynyl)phenyl)ethynyl)phenyl)hexa-3-en-1,5-diyne (trans-3). A benzene solution of trans-1,2-dichloroethylene (20 mg, 0.2 mmol) and 1,3-di-tert-butyl-5-(2-(4-(2-(4-ethynylphenyl)ethynyl)phenyl)ethynyl)benzene (22 mg, 0.53 mmol) was added to a mixture of Pd(PPh3)2Cl2 (22 mg, 0.03 mmol), CuI (7.8 mg, 0.04 mmol), and n-BuNH2 (53 mg, 0.72 mmol) in 10 mL of deaerated benzene which was cooled at -78 °C. The mixture was allowed to warm to room temperature and stirred for 6 h. After evaporation, the residue was purified by silica gel column chromatography (eluent, hexane/chloroform (90/10)) to give trans-3 as a white powder (18.4 mg, 10.8%). 1H NMR (270 MHz, CDCl3): 7.55-7.45 (m, 16H), 7.42-7.39 (m, 6H), 6.31 (s, 1H), 1.34 (s, 36H). 13C NMR (67.5 MHz, CDCl3): 150.8, 131.5, 131.5, 131.4, 125.8, 123.6, 123.3, 123.0, 122.6, 122.4, 121.8, 120.8, 94.9, 92.6, 91.4, 90.3, 90.0, 87.8, 34.9, 31.4. MALDI-TOFMS (m/z) [M + H]+ calcd for C66H60, 853.47; found, 853.48. Results and Discussion Steady State Absorption and Fluorescence Spectra. The para-substituent effect of the phenylacetylenyl group is signifi-
cant. The UV absorption spectrum of trans-BEE in benzene is reported to exhibit an absorption band around 300-360 nm.5 The absorption spectra of both trans-BEE and the tert-butyl substituted analogue trans-1 and phenylacetylenyl and (phenylacetylenyl)phenylacetylenyl substituted enediynes trans-2 and trans-3 in benzene solution are shown in Figure 2a. The spectra of trans-1 is slightly red-shifted compared to that of trans-BEE, presumably due to the difference in electron-donating ability of H and the tert-butyl group. Compared with the absorption spectrum of trans-1, the spectrum of trans-2 is dramatically redshifted to 420 nm and its shape is also changed, indicating that the π-electrons of the para-substituted phenylacetylenyl group are delocalized into the core enediyne moiety. There is also a shoulder at 395 nm in the spectrum for trans-2 that seems to correspond to shoulders at 352 nm in the spectrum for BEE and 360 nm in the spectrum for trans-1. Interestingly, the absorption spectrum of trans-3 showed only slight red-shifting compared to that of trans-2 despite further extension of the π-conjugate system, while the extinction coefficient increased significantly to as high as 122000 M-1 cm-1. Some similarities and differences were also noted for the corresponding cis isomers. Their absorption spectra can be seen in Figure 1b. Although the extinction coefficients of the cis isomers are almost half as much as the values for the trans isomers, the spectral shape of cis- and trans-1 are quite similar to each other. The shapes of the spectra for cis-2 and cis-3, however, are different from those of the corresponding trans isomers. The absorption band of cis-2 consists of the superposition of a broad band around 330-420 nm similar to that seen in trans-1 and a band peaking at 315 nm, probably arising from partial localization of the π-electron system at the phenylacetylene moiety in the cis isomer. A similar trend was observed in cis-3; the absorption band consists of two independent parts, a broad band around 350-420 nm and a strong band peaking at 336 nm, which may be ascribed to the band of the origo(phenylacetylene) unit.14-16 The energies for the lowest transition were calculated to be 81, 79, 71, and 70 kcal/mol for both trans and cis isomers of BEE, 1, 2, and 3, respectively. The phenylacetylene substituents also have an impact on the fluorescence spectra, which are shown for the trans and cis isomers in parts c and d of Figure 2, respectively, with some relevant data also presented in Table 1. trans-BEE is reported
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TABLE 1: Spectroscopic, Photophysical, and Photochemical Data (Benzene Solution, 298 K)
trans-BEE trans-1 trans-2 trans-3 cis-BEE cis-1 cis-2 cis-3
λmax (abs) /nm
max/M-1 cm-1
λmax (fl)/nm
Stokes shift/cm-1
328, 352 334, 360 365, 395 373, 405 328, 352 333, 359 315, 365, 395 336, 375, 405
40100, 27100 52200, 35200 79700, 50000 122000, 66000 21100, 12600 23300, 13900 62700, 38200, 19500 108800, 63000, 25000
362, 382 370, 392 411, 436 423, 448 364, 387 373, 394 413, 437 423, 448
780 750 990 1050 940 1050 1100 1050
to exhibit a fluorescence band with λmax ) 362 and 381 nm and Φf ) 0.42 in benzene under argon.5 The fluorescence of trans-1 is similar to that of trans-BEE but more intense (Φf ) 0.61). The fluorescence spectrum of trans-2 is more than 40 nm red-shifted to 411 and 436 nm and exhibits a long tail at low energy around 560 nm. The fluorescence quantum yield of trans-2 (Φf ) 0.61) is the same value as trans-1, and the singlet lifetime is similar: τs ) 0.69 ns for trans-1 and 0.61 ns for trans2, respectively. The fluorescence spectrum of trans-3 is slightly (11 nm) red-shifted compared to that of trans-2 and exhibits a long tail at low energy around 560 nm which is almost the same as that of trans-2. The fluorescence quantum yield of trans-3 (Φf ) 0.48) is lower than those of trans-1 and trans-2, and the singlet lifetime is shorter (0.47 ns), probably due to the promotion of nonradiative decay processes. The Stokes shift values (780, 990, and 1050 cm-1 for trans1, trans-2, and trans-3, respectively) only slightly increase with expansion of the phenylacetylene chain, indicating that the structural differences between the ground state and the excited singlet state are not very significant among 1-3. In the ground state, both planar and perpendicular conformations at the phenylacetylene moiety should exist because of free rotation of the single bonds, whereas in the excited singlet state an immediate structural change to the planar conformation takes place that results in fluorescence.17,18 In this case, the enediyne having a longer phenylacetylene chain could cause more significant conformational changes in the excited singlet state,15 resulting in a greater value of the Stokes shift than that observed for the shorter chain. However, the calculated values of the Stokes shift using the lowest energy transition band of the absorption spectra is not very much different, suggesting that the conformation of the fluorescent state should be almost similar among 1-3. Fluorescence lifetimes of the examined molecules are also listed in Table 1. All compounds exhibited single exponential decay, and the wavelength dependence of time constants was not observed, indicating a single emitting state in all cases. Excitation Spectra. Excitation spectra provided further information about the substituted endiynes. Figure 3 shows fluorescence excitation and absorption spectra. All of the trans isomers and cis-1 exhibited similar absorption and excitation spectral shapes, whereas the excitation spectra of cis-2 and cis-3 did not agree with their absorption spectra, especially at the shorter wavelength region. This result indicates that interaction between the phenylacetylenyl units and the enediyne chromophore is not significant in cis-2 and cis-3. In addition, the energy transfer from excited phenylacetylenyl units to the enediyne chromophore does not occur efficiently, despite the fact that the phenylacetylenyl units are substituted in the para position. In other words, the excitation at 315 and 336 nm for cis-2 and cis-3, respectively, can produce locally excited states at the phenylacetylenyl units that mainly decay nonradia-
lowest singlet energy/kcal mol-1
Φf
τS/ns
81 79 71 70 81 79 71 70
0.42 0.61 0.61 0.47 0.31 0.39 0.60 0.42
0.69 0.61 0.52 0.88 1.13 1.42 1.28
Φtfc
Φcft
τT/ns
0.27 0.19 0.03 0.02
0.37 0.49 2.45 2.86 0.37 0.48 2.47 2.86
0.17 0.10 0.007 0.004
Kq/109 M-1 s-1 4.22 3.25 2.88 4.16 2.94 3.04
tively.19-21 On the other hand, distinctive absorption bands for phenylacetylenyl units cannot be observed in the spectra for trans-2 and trans-3, indicating that the π-electron system delocalizes over the whole molecule in these isomers. Photoisomerization. Aromatic enediynes are known to undergo a variety of photochemical processes including trans-cis photoisomerization. The changes in the absorption spectra observed upon irradiation with 365 nm light in benzene solution under argon are shown in Figure 4, and some relevant data are gathered in Table 1. Since the isosbestic point can be observed during the photoirradiation (for 2 and 3 only) and the spectral shapes at the photostationary state of the trans-starting and cisstarting sample (solid line) are almost identical, the trans-cis photoisomerization in the examined molecules is a clean process
Figure 3. Absorption spectra (dashed-line) and fluorescence excitation spectra (solid line) of (a) trans-1, (b) trans-2, (c) trans-3, (d) cis-1, (e) cis-2, and (f) cis-3 in benzene under argon.
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Figure 4. Change in the absorption spectra of (a) cis-1, (b) cis-2, and (c) cis-3 upon irradiation with 365 nm light in benzene under argon. Inset shows change in the absorption spectra of the corresponding trans isomer upon irradiation with 365 nm light under the same conditions.
and the photoproducts are either the trans or cis isomers. The lack of observation of isosbestic point for BEE and 1 is due to the similarity of the absorption spectra of trans and cis isomers with smaller extinction coefficient around 300-350 nm in cis isomer. From the absorption spectra of pure trans and cis isomers, the trans/cis ratio at the photostationary state ([trans]/ [cis])pss can be calculated to be 48/52, 70/30, and 73/27 for 1, 2, and 3, respectively. This indicates that the photoismerization from trans isomer is more difficult than that from cis isomer in 2 and 3. The quantum yields for trans-to-cis photoisomerization (Φtfc) of trans-1 are calculated by the equation
Φtfc ) Φcftεcis /εtransx([trans]/[cis])pss
(1)
where εcis and εtrans are the molar extinction coefficients at 365 nm for cis-1 and trans-1, respectively, and Φcft is the quantum yield for cis-to-trans photoisomerization obtained at a very early stage. The Φtfc value obtained for trans-1 (0.10) is on the same order as that obtained for trans-BEE (0.19), whereas those for trans-2 (0.007) and trans-3 (0.004) are considerably smaller. The much lower Φtfc values for trans-2 and trans-3 may be due in part to steric interactions with surrounding solvent molecules during rotation around the CdC double bond of the phenylacetylene enediyne. It is more likely, though, to be related to the promotion of alternative nonradiative decay processes due to the lower excited state energy of trans-2 and trans-3 or the presence of an increasing number of possible reaction pathways from the emitting species to a nonemitting perpendicular species in the excited state.19 Photoisomerization could thus proceed mainly via the excited triplet state (see below). The quantum yield for nonradiative decay other than photoisomerization (Φnr) can be calculated using the Φf and Φtfc values
Φnr ) 1 - (Φf + Φtfc)
(2)
The calculated Φnr values are 0.19, 0.38, and 0.52 for trans-1, trans-2, and trans-3, respectively. Similar values were obtained for the cis isomers. These values support above statements that nonradiative decay processes are promoted with increasing chain length. Transient Absorption Spectra. Transient absorption spectra were collected to gain further insight into the excited states of compounds 1-3. Upon 355 nm laser excitation, each one of the trans and cis isomers gave a clear transient absorption spectrum (Figure 5). The observed transient species for all compounds were quenched by oxygen with a rate constant of (2.9-4.2) × 109 M-1 s-1 (Table 1), indicating that the observed transients are assignable to the triplet state.22 The transient absorption spectra of the trans and cis isomers are quite similar to each other (Figure 5). In addition, the triplet lifetimes (τt) are almost the same for the trans and cis isomers (Table 1), suggesting that both undergo intersystem crossing to the triplet state where the planar triplet state (3trans* and 3cis*) and the perpendicular triplet state (3p*) are equilibrated. The potential energy of 3p* does not change with introduction of substituents on the phenyl ring,22 and thus the energies of 3p* are nearly the same among all examined compounds (Figure 6), whereas the energies of the planar conformations 3trans* and 3cis* depend on the substituent. The energies of 3trans* and 3cis* of 1 and BEE are nearly the same based on the similarities in their T-T absorption spectra. The energies of 3trans* and 3cis* of 2 and 3, however, are expected to be lower than those of 1 based on the fact that the triplet lifetimes of 2 and 3 are 5-6 times longer than that of 1. The lower planar triplet energies in 2 and 3 probably promote a nonradiative deactivation process from the planar triplet state (Figure 6), and therefore the quantum yield of isomerization is much lower in 2 and 3 than it is in BEE and 1. This result indicates that the photoisomerization of 1-3 partially proceeds from the excited triplet state. Since the deactivation rate constant from 3trans* to the ground state trans isomer is reasonably estimated to be the same as that from 3cis* to the ground state cis isomer, the equilibrium constant between the planar triplet state (3trans* and 3cis*) and 3 p* can be estimated using the observed triplet lifetime. The triplet lifetime (τt) can be described in the equation
τt ) (1 + K)/(Kkd + kd′)
(3)
where kd and kd′ are the rate constants for deactivation from the planar triplet state and 3p*, respectively, and K is the equilibrium constant between the planar triplet state and 3p*. The kd and kd′ values were previously estimated to be 2 × 104 s-1 and 2 × 107 s-1, respectively.22 Using this information, the equilibrium constants K were determined to be 0.11, 0.020, and 0.020 for 1, 2, and 3, respectively (Table 2). Furthermore, the proportion of the planar and perpendicular excited triplet species (3trans* and 3cis*) and 3p* was roughly estimated to be 89:11, 98:2, and 98:2 for 1, 2, and 3, respectively (Table 2). These values support that the energies of planar triplet state in 2 and 3 are lower than that of 1 and the quantum yield of isomerization is much lower in 2 and 3 due to the promotion of a nonradiateve process from the planar triplet state. Potential Energy Surfaces. On the basis of the experimental results, the potential energy surfaces of 1-3 were estimated and are depicted in Figure 6. The potential energy surfaces for photoisomerization depend on the length of the phenyl acetylenyl chain. In the triplet state, the three conformers 3trans* and 3 cis* and 3p* are equilibrated and the photoisomerization may take place from the 3p* conformer. In particular, the energies
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Figure 5. Transient absorption spectra of (a) trans-1, (b) trans-2, (c) trans-3, (d) cis-1, (e) cis-2, and (f) cis-3 in benzene under argon.
of the planar conformations of 2 and 3 are lower than that of 1, while those of 3p* are independent of the length of the phenyl acetylenyl chain. The result is a higher activation energy for excitation from the planar conformations to 3p* in 2 and 3 and thus a decrease in the quantum yield of the photoisomerization process. In addition, deactivation processes related to the phenylacetylene group may also be a source of nonradiative decay.19 Conclusion In summary, a series of phenylacetylenyl- and (phenylacetylenyl)phenylacetylenyl-substituted aromatic enediynes 1-3 were synthesized as pure trans and cis isomers and their photochemical properties studied. With expansion of the π-electron system, the absorption spectra red-shifted and the molar extinction coefficients dramatically increased in both the trans and cis isomers. The absorption spectra of cis-2 and cis-3 consist of two distinctive absorption bands of which the band at the shorter wavelength region is ascribed to the substituted phenylacetylenyl units. The fluorescence excitation spectra of both cis-2 and cis-3 do not agree with those of the absorption spectra, indicating
Figure 6. Potential energy surfaces for the photoisomerization of 1-3.
Photochemistry of Substituted Aromatic Enediynes TABLE 2: Equilibrium Constants between the Planar (3trans* and 3cis*) and Perpendicular Triplet States (3p*) and the Population Ratio of the Planar and Perpendicular Triplet States in Benzene under Argon BEE 1 2 3
K
{[3p*]/[3trans*] + [3cis*]}
0.16 0.11 0.020 0.020
16/84 11/89 2/98 2/98
that the excited state of phenylacetylenyl units in cis-2 and cis-3 mainly decay nonradiatively. The fluorescence quantum yields of examined molecules were high, even for the cis isomers (Φf ) 0.39-0.61). The fluorescence decay of each one of the compounds was analyzed as a single exponential and the wavelength dependence of time constants was not observed, indicating the presence of a single emitting state in all cases. All isomers exhibited mutual cis-trans photoisomerization. The quantum yield of both trans-to-cis and cis-to-trans photoisomerization considerably decreased in 2 and 3, presumably because of an increase in the number of photochemical processes that yield nonfluorescent excited species. Furthermore, three energy minima exist in the excited triplet state where the energy of the planar conformation decreased with extension of the phenyl acetylenyl chain, resulting in the promotion of nonradiative processes without conformational change. Acknowledgment. This work was supported by a Grant-inAid for Scientific Research in a Priority Area “New Frontiers in Photochromism (No. 471) and (No. 19550176) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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